Observation of red emission in wurtzite ZnS nanoparticles and the investigation of phonon modes by Raman spectroscopy

Wurtzite ZnS nanoparticles are synthesized by a low temperature chemical method using ethylene glycol. Photoluminescence studies reveal a new red emission along with strong UV and blue emission known to occur from nanocrystalline ZnS. An energy level diagram involving oxygen trap levels and interstitial sulphur ions is proposed to explain the origin of the observed emission peaks. Micro-Raman spectroscopic studies indicate that the optic modes get softened whereas the second order longitudinal acoustic (LA) phonon mode get hardened in the nanocrystals. The analysis is done by taking into account the effect of the small particle size of nanoparticles as well as the existence of defects resulting in strain in the lattice.

► Observation of emission in red region from Wurtzite ZnS nanoparticles. ► XPS indicates replacement of S2− ions by O2− ions at random points in the lattice. ► Interpretation of red emission on the basis of defect levels by O2− ions. ► Analysis of phonon modes using Raman spectroscopy. ► Simultaneous observation of optic mode softening and acoustic mode hardening.